[WebAssembly] Don't use setRequiresStructuredCFG(true).

While we still do want reducible control flow, the RequiresStructuredCFG
flag imposes more strict structure constraints than WebAssembly wants.
Unsetting this flag enables critical edge splitting and tail merging.

Also, disable TailDuplication explicitly, as it doesn't support virtual
registers, and was previously only disabled by the RequiresStructuredCFG
flag.

llvm-svn: 261190